(1) Field of the Invention
The present invention relates generally to transmission lines and more particularly, to broadband spiral transmission line power splitters.
(2) Description of Prior Art
The use of ¼ wavelength 90 degree power splitters is well known in the prior art. For example, FIG. 1a and FIG. 1b show a prior art ¼ wavelength 90 degree power splitters comprising two “hot” lines L1 and L2 that are two unbalanced transmission lines running side-by-side for a ¼ wavelength distance on a dielectric surface sheet 108. The lines share a common ground plane and have a characteristic impedance, Z0, to ground, which is usually 50 ohms. Also referring to FIG. 1c, the ground plane consists of a top metal ground plane gp1 and a bottom metal ground plane gp2. The two ground planes are held together by metal sides 110. If the power splitter is made symmetrical about a horizontal plane 2014 through the centers of the lines L1 and L2, so that the thickness 2001 of the space above the lines L1 and L2 equals the thickness 2002 of the space below the lines, and the unused space inside the cavity 2003 formed by the two ground planes gp1, gp2 and the two metal sides 110 is filled with the same dielectric material as dielectric 108, then the structure is strip line.
If as shown in FIG. 1b, thickness 2001 is appreciably larger than thickness 2002 and dielectric 108 only exists between the lines L1 and L2 and ground plane gp2, the structure is microstrip. In this case, there is little or no coupling between ground plane gp2 and lines L1 and L2, and thus ground plane gp2 serves more as a shield than a ground plane.
A first line L1 has a first end 100 as an input (port relative to ground) and a second end 102 as an output (port relative to the ground). A second line L2 has a first end 104 and is coupled to power of the first line L1 wherein the amount of power coupled thereto increases as the coupling between the two lines increases and the separation between the two lines decreases. Even higher degrees of coupling occur if the lines L1 and L2 start to overlap each other without touching. The coupled output port relative to ground or first end 104 of the second line L2 is on the same end as the input end 100 of the first line L1. A second end 106 of the second line L2 is an isolation or dump port relative to ground, and terminates to ground in Z0. Ideally, when all of the ports are properly matched to Z0, the resultant phase difference between the outputs of the first and second lines L1 and L2 is 90 degrees, and all of the input power is divided between the two output ports, with none of it going to the isolation port 106.
If the power splitter is made with microstrip, then access to the ports at the ends of the lines are made by placing connectors on the outside of the splitter located below the ends and below the ground plane gp2, such as for example at locations 2006 and 2007 for respective ports 104 and 106 for line L2. If the splitter is made with strip line, to maintain symmetry, the locations are moved up the sides 110 of the splitter to lie coincident with axis of the lines L1 and L2, such as for example locations 2008 and 2009 for respective ports 104 and 106 of lines L2. Since the two adjacent ends of the two lines L1 and L2 are usually very close to each other, the locations are spread out to allow two corresponding connectors to be placed adjacent to each other, and added lengths of transmission line are used to connect the connectors to the ends of the lines.
If the coupled power is less than one half the input power, the power splitter is also called a directional coupler, because the coupled power depends on the direction of the wave travelling along the line L1. When power is applied to input port 100, it travels in a forward direction from the port to output port 102 and some of it is coupled to line L2 at coupled port 104. No power is coupled to isolation port 106. If instead power were inserted at output port 102, it would flow backwards from the output port 102 to input port 100 and the roles of the coupled and isolation ports would become reversed. Power from the backward travelling wave would couple to port 106, which is now a coupled port, and none of the power would couple to port 104 which is now an isolation port. In general, if any given port of any first line serves as an input port, the opposite port of the first line will be an output port, the adjacent port on the second line will be a coupled port, and the opposite port of the adjacent port on the second line will be an isolation port.
Because the widths of lines L1 and L2 and their separation 2005 are much smaller than their λ/4 length, the size of the power splitter is primarily determined by the length of the lines L1 and L2. At lower frequencies this length can become excessive. One well known method to reduce the length of the lines is meandering. The lines L1 and L2 can be meandered about a center line between the two lines L1 and L2. FIG. 1d shows the prior art splitter as FIG. 1a where the lines L1 and L2 are separated less and are meandered. Reducing the line widths and their separation further would allow more meander cycles and a much smaller device. Care must be taken to avoid allowing adjacent section of a line, e.g. 1002 and 1003 from coming too close to each other. If the separation 1001 becomes too small, broadside coupling between the sections 1002 and 1003 occurs, and a second mechanism of power transfer between the line ends is introduced along the straight line direction 1004 between the line ends. Only the primary mechanism of power transfer along the transmission line paths of L1 and L2 should be allowed.
The coupling for the ¼ wavelength 90 degree splitter is frequency dependent wherein maximum coupling occurs every ½ wavelength starting at ¼ wavelength. Nulls in coupling occur every ½ wavelength starting at zero wavelengths between the maximum points. Normally the splitter is used at ¼ wavelength. Because the maximum at ¼ wavelength is between nulls at zero and ½ wavelengths, the ¼ wavelength 90 degree splitter is narrowband as far as constant coupling with frequency is concerned. To obtain broader bandwidth, additional ¼-wavelength sections of two coupled lines are added to the splitter, making it more complex.
A disadvantage of the ¼ wavelength 90-degree splitter of the prior art is that its length results in narrowband performance. In the alternative, a bifilar spiral appears lengthless in the radiation domain above a cut-in frequency. Different circumferential lengths radiate at different frequencies, so that radiation always occurs from a circumferential electrical length of one wavelength. When used as a transmission line, a length of a bifilar spiral is difficult to define. Starting at a center feed point of the bifilar spiral and moving outwardly in a circular direction the filar of the spiraled transmission line making up the bifilar spiral eventually couples broadside to itself (via the other filar) at the next turn and then at each succeeding turn with the circumferential length between each turn increasing. If the one-dimensional transmission line is of low Z0 and highly coupled, the filar of the transmission line becomes highly coupled to itself, and the transmission line starts to appear to be a two-dimensional instead of a one-dimensional transmission line. Power is transferred in the circumferential direction via the two line transmission line and in the radial direction via broadside coupling. This differs from the λ/4 90 degree splitter where power transfer is only via the two line transmission line. Thus the two-dimensional size and different circumferential lengths may allow broadband behavior.
U.S. Pat. No. 6,133,891, hereby incorporated by reference, describes spiral transmission lines. This patent describes two spirals that are crossed to form two crossed transmission lines comprising elements for feeding and matching a quadrifilar helix. The two transmission lines are approximately balanced and are of constant or smoothly changing Z0 with length except for the last ½ of a turn of any given element on the outermost circumference. For a given transmission line length, the given filar has filars on both of its sides. However for the last ½ turn, the given filar has only one opposite filar, which is on the side closest the feed points (the central region) of the spiral. This increases the Z0 of the transmission line along this ½ turn causing a mismatch.
The mismatch shows up as a small increased antenna mismatch when the transmission line is used to feed and match the antenna. If the width of the filar is increased in the area of the ½ turn to increase capacitance to the opposite filar, the Z0 between the ½ turn of filar and its opposite transmission line filar decreases back to normal. But now the capacitance between the opposite filar and its two surrounding opposite filars, which includes the widened ½ turn of filar, becomes larger than normal resulting in its Z0 becoming lower than normal. Thus, this attempt at fixing the first mismatch of the ½ turns of filar creates a second mismatch.